This invention relates in general to semiconductor processing and, more particularly, to an improved method for processing silicon-on-sapphire semiconductor devices to produce an improved gate structure.
In the normal processing of a silicon-on-sapphire semiconductor device, the classic method of forming the island utilizes an anisotropic etch. This etch is usually done with a potassium hydroxide (KOH) solution which, by its nature, will produce islands havng sloped sides wherein the base of the island is longer and broader than the top surface thereof. However, when this island is utilized to form a semiconductor device without any further treatment, it has been noted that the gate dielectric breakdown voltage is consistently lower than in edgeless devices. The cause of this low gate dielectric breakdown voltage was investigated and found to have occurred at the upper edge of the island where a peak or point was produced as a result of the anisotropic etch. For a more detailed discussion of the structure near the top edge of the island, the reader is directed to an article entitled "The Study of Microcircuits by Transmission Electron Microscopy" by W. E. Ham et al. appearing in RCA Review, Vol. 38, September 1977, pp. 351-389. Particular attention is directed to FIGS. 12-15 wherein a high magnification cross section transmission electron microscopy shows an image of an island edge after an anisotropic etch. It was theorized that the formation of the sharp edge otherwise known as "interface tilt", appearing at the upper edge of the silicon island, also causes the oxide formed around this point to be thinner than that in other parts of the island. Consequently, the low gate breakdown voltage would most likely occur at the top edge adjacent the point.
One method that was tried, in order to remove the objectionable point at the top edge of the silicon island, was to heavily oxidize the side of the island. However, since thermal oxidation of a given volume of silicon produces approximately twice that value of oxide, it will be seen that to produce an oxide thickness of approximately 1000 angstroms, only 500 angstroms of silicon would be oxidized. This would not be enough to remove the objectionable point and in any event would introduce still another discontinuity. A heavy oxidation of this sort would only appear on the side of the silicon island and would provide a noticeable and undesirable separation or lifting of silicon oxide from the sapphire surface.